DC/DC voltage regulator with automatic current sensing selectability for linear and switch mode operation utilizing a single voltage reference

ABSTRACT

A switch-mode DC/DC converter ( 101 ) and linear low drop out (LIDO) DC/DC regulator ( 105 ) are connected in parallel to drive a single load Both regulators share a common voltage reference, feedback network ( 103 ), input supply and output such that the regulated voltage is identical during each mode of operation. During heavy loads the switch-mode regulator ( 107 ) is in operation and the linear regulator is disabled for the highest efficiency possible. Conversely at light loads the linear regulator is in operation with the switch-mode regulator ( 107 ) disabled, also maximizing the efficiency. Each regulator senses load current ( 131 ) to automatically transition between the appropriate voltage regulators at fixed load current levels. The presented invention also includes a make before break transition scheme of the voltage regulators ( 103 ) to minimize the voltage transients.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a US National Phase Application of PCT/U2006/003267,filed Feb. 5, 2004, to which the benefit of priority is claimed and theentirety of which is incorporated herein by reference.

TECHNICAL FIELD

This invention relates to a voltage regulator that has both a linear lowdrop out (LDO) regulator and a switch-mode converter, and moreparticularly, to a voltage regulator that can select between the LDOregulator and switch-mode converter based upon load current.

BACKGROUND

A DC/DC voltage regulator is operative to maintain a level outputvoltage despite variations in power supply voltage or current drawn by aload. As one example, a step down converter may take a relativelyunstable input voltage and maintain a desired level output voltage thatis nominally lower than the input voltage. Specifically, a step downconverter may receive as an input a voltage in the range of 2-10 voltsand output a level voltage of (for example) 1.8 volts.

Many portable devices require a steady voltage supply such as thatprovided by a DC/DC voltage regulator. Further, the advent of portableelectronics and the need for longer battery life requires new types ofvoltage regulators. These DC/DC voltage regulators need to be efficientwhile operating in both low current and high current load conditions.For example, handheld electronics such as PDA's and cell phones nowrequire high efficiency at varying loads (such as standby and activemodes) to extend battery life. The standby mode requires a very lowamount of current to operate. Only critical systems and volatile memoryneed to be powered to constantly refresh and maintain the data in thedevice. Because of these requirements, new voltage regulator schemeshave been developed that are very efficient at all current levels.

For example, a pulse width modulated (PWM) switch-mode regulator is anefficient regulation scheme during heavy loads. It offers highefficiency, low output voltage ripple, good line and load regulation.However at light loads the PWM regulator has poor efficiency.

At light loads, a pulsed frequency modulation (PFM) switch-moderegulator is commonly used due to its high efficiency. However the largeoutput voltage ripple, poor line and load regulation inherent to PFMprecludes its use in many systems. Thus, a low quiescent current LDOregulator is desirable in these systems. The LDO regulator offersrelatively good light load efficiency, low output voltage ripple, andgood line and load regulation, but at heavy loads the efficiency is farbelow that of switching regulators.

Combining a PWM switch-mode regulator and a linear LDO regulator inparallel offers the high efficiency and good output voltage regulationrequired by many portable battery powered systems. Switch-mode andlinear regulators have been paralleled in the prior art, but theregulation voltages have been slightly different, and the control schemeof the regulators are very basic. These types of systems do not have theoptimal efficiency and the output voltage regulates at two differentvoltages making the load regulation poor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a DC/DC voltage regulator formed in accordance withthe present invention.

FIG. 2 is a flow diagram illustrating the control methodology for theregulator of FIG. 1.

FIG. 3 illustrates a DC/DC voltage regulator operating in switch mode.

FIG. 4 illustrates a DC/DC voltage regulator operating in linear mode.

DETAILED DESCRIPTION

This present invention discloses a DC/DC voltage regulator system thatintegrates both a switch-mode (PWM) regulator and a linear regulatorwhile using a single voltage reference. Further, a control scheme isdescribed to implement an automatic “make before break” transitionbetween the two modes to minimize switch over transients.

In the following description, numerous specific details are provided,such as the identification of various system components, to provide athorough understanding of embodiments of the invention. One skilled inthe art will recognize, however, that the invention can be practicedwithout one or more of the specific details, or with other methods,components, materials, etc. In still other instances, well-knownstructures, materials, or operations are not shown or described indetail to avoid obscuring aspects of various embodiments of theinvention.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure, orcharacteristic described in connection with the embodiment is includedin at least one embodiment of the present invention. Thus, theappearance of the phrases “in one embodiment” or “in an embodiment” invarious places throughout this specification are not necessarily allreferring to the same embodiment. Furthermore, the particular features,structures, or characteristics may be combined in any suitable manner inone or more embodiments.

As seen in greater detail below, a linear regulator is a relativelysimple implementation of a DC/DC voltage regulator where a pass device,usually a transistor, is connected between an input node and the outputnode. This pass device is controlled by a linear block, usually an erroramplifier. The error amplifier senses the voltage at the output by wayof feedback. If the voltage at the output drops, the amplifier signalsthe pass device to deliver more current. Likewise, if the voltage at theoutput increases the error amplifier signals the pass device to deliverless current.

In contrast, a switching regulator of the present invention consists of:(1) a PWM controller 129, (2) two controlled pass devices connected inseries between a supply voltage and ground, (3) an inductor which isconnected to the drains of the pass devices and an output node, and (4)a capacitor connected between the output node and ground and after theinductor. The capacitor side of the inductor is the filtered outputnode. The other side of the inductor (connected to the pass devices)switches from the supply voltage and ground (as controlled by the PWMcontroller). The PWM regulator in this work operates in current mode,which is described as follows.

By modulating the switching of the pass devices, the inductor will buildup energy and discharge it into the capacitor and the load. The passdevices are operated in a periodic manner delivering current to theload. A control element, usually an error amplifier, compares the outputvoltage to an internal reference voltage and modulates the PWM signalaccordingly, depending if the output voltage is higher or lower than thevoltage reference.

The present invention uses these two types of voltage regulators and onefeedback network to regulate a single constant voltage. The systemmaintains the output voltage regardless of changes in load current atthe output. A current mode switching regulator is used to regulate theoutput during heavy loads. A linear LDO regulator is used to regulatethe output during light loads. Since both of the voltage regulatorsutilize the same feedback network, the output voltage for all loads isregulated to the desired value no matter if the voltage regulator is inlinear or switching mode.

A block diagram of the system 101 can be found in FIG. 1. The systemuses an overlapping regulator selection scheme to automatically switchbetween the modes of operation. This selection scheme is necessary tominimize voltage excursions during load transients. A flow chart of theautomatic selection scheme is seen in FIG. 2.

The system includes a feedback circuit 103, a LDO regulator 105, a PWMregulator 107, a mode selection circuit 109, a LDO current sensingcircuit 111, and an oscillator 113. The LDO regulator 105 and the PWMregulator 107 operate according to the same principles already describedabove.

The feedback circuit includes a LDO error amplifier 115 and a PWM erroramplifier 117. As described above with respect to the PWM regulator, theoutput pin SW of the PWM regulator 107 is connected through an inductorL in series to the output node V_(out). Further, the output node isconnected to ground through a capacitor. The OUT pin of the system 101is the output from the LDO regulator 105. The OUT pin is connecteddirectly to the output node V_(out).

The feedback pin FB is used to monitor the output node V_(out) through aresistive voltage divider. The voltage on feedback pin FB is provided tothe feedback circuit 103. The signal is independently processed by theerror amplifiers 115 and 117. These error amplifiers provide anindication to the respective LDO and PWM regulators as to the voltage onthe output node V_(out). Specifically, the output of the LDO erroramplifier 115 is used to selectively control the LDO regulator 105 toensure that the correct current flows through a pass device 119 of theLDO regulator 105 to ensure a stable voltage on the output node.

Thus, as seen in FIG. 1, the switch mode PWM regulator and linearregulator both use a common feedback circuit and a single voltagereference (the voltage on the feedback pin FB). The PWM regulator'serror amplifier 117 is connected into the PWM comparator 125 such thatthe PWM regulator 107 is in current mode configuration. Because of thecommon feedback and voltage reference, both of the PWM and LDOregulators will output the same voltage.

The PWM generator is a current mode switching regulator that comparesthe inductor current with a compensation voltage that is set by theoutput voltage and the output load current. The PWM controller 129regulates the load current and the output voltage via the feedbackcircuit 103. The PWM error amplifier 117 compares a known voltage withthe output voltage. The output of the PWM error amplifier 117 isconsidered the compensation (COMP) node. This node is usually smoothedwith external components, usually a capacitor and resistor (as seen inFIG. 1).

The current mode PWM controller 129 modulates the PWM signal bycomparing the compensation node COMP to the inductor current informationacquired by the current sense amplifier 131. As the output currentincreases, the COMP voltage also increases. The COMP node is clamped toprovide a current limit for the system, such that the COMP voltage willnot increase past a certain point.

The invention automatically selects between switch-mode PWM or LDO moderegulation by monitoring the load current. If a condition exists suchthat the switching PWM regulator 107 is turned on and the LDO regulator103 is disabled, this means that the load current is higher than a “lowcurrent limit.” The switch-mode PWM regulator 107 is thus operating innormal current mode operation. Under these conditions, the upper (highside) switch 121 is turned on cycle by cycle by a periodic signal drivenin this system by the oscillator 113 and control logic 129. Thecompensation voltage on the COMP pin compared to the current in theinductor L, via the current sense amplifier 131, while the high sideswitch 121 is on. When these two voltages are equal, the loop issatisfied, the PWM comparator 125 is turned on, the upper switch isturned off 121, and the lower switch 123 is turned on, and the outputvoltage is in regulation.

As the current load drops below the low current limit, a signal from thelow current limit is given to the selection logic 109 to turn on the LDOregulator 105. With the LDO regulator 105 turned on, current isdelivered to the load by the linear pass device 119. This bypasses theinductor L and the current in the inductor L will fall to zero. Whenthis occurs, the zero cross comparator 127 will signal to disable theswitching PWM regulator 107. When the inductor current reaches zero, thePWM regulator 107 is disabled, and a buffer will charge the voltage ofthe compensation node COMP to a set voltage that is in the operatingrange of the error amplifier. This will ensure that the switching PWMregulator 107 will start in a high current state. With the PWM regulator107 is disabled, the LDO will solely be delivering the current to theload. The block diagram for the switching regulator with low currentsensing comparators is seen in FIG. 3.

The transition from low current LDO operation to high current switchingregulator operation is triggered by detecting a high load current in theLDO. The LDO current sense circuit 111 will signal a high current to themode selection circuit 109 and turn on the switching PWM regulator 107.Since the switching PWM regulator 107 will not enable instantaneously,the “buffer disable signal” is delayed a short time to allow for theswitching PWM regulator 107 to power on. The loop of the switching PWMregulator 107 now tries to regulate the loop. Since the compensationnode is pre-charged, the inductor current has to ramp up to a finitevalue in order to trigger the PWM comparator 125. Once this occurs thesystem is running and current is delivered from the switching PWMregulator 107 and the LDO regulator can be safely disabled. Thisminimizes voltage excursions during transitions. The high currentsensing techniques and buffer can be seen in the linear regulator blockdiagram in FIG. 4.

Returning to FIG. 2, the process above can be seen in flow diagram form.At block 201, the regulator system 101 is enabled (turned on). In thisembodiment, the default start up condition is to use the LDO regulatorto supply current to the load, and thus at block 202, the LDO regulator105 is turned on while the PWM regulator is turned off. Further, atblock 203 the compensation pin COMP is buffered to the FB pin.

Note that the feedback voltage is selected for this buffered voltage sothat a separate voltage source is not required and the system can run ata lower quiescent current, meaning higher efficiency at low loads. Thisvoltage, however, may be selected at any voltage within the range ofoperation of the error amplifier.

Then, at block 205, the current through the LDO regulator 105 iscontinuously monitored to determine if it is above a predeterminedthreshold level (A). If the current rises above the threshold A, then atblock 207, the PWM regulator 107 is enabled and the COMP pin isunbuffered to pin FB.

Next, at box 209, once the PWM comparator 125 is triggered (indicatingthat the PWM regulator 107 is operating correctly), at block 215, theLDO regulator 105 is disabled.

Further, at block 211, the current through the inductor L (as measuredby a current sense amplifier 131) is monitored cycle by cycle while theupper FET 121 is turned on to determine if it is below a secondpredetermined threshold level B. If the current falls below thethreshold B, then at block 212, the LDO is enabled. At block 213, theswitching regulator 107 will remain on until the inductor currentreaches zero, and if so, control returns to block 203. In other words,the transition from the PWM regulator 107 to the LDO regulator 105 doesnot occur until the inductor has reached zero.

FIG. 3 illustrates the system operating in PWM mode while FIG. 4illustrates the system operating in LDO mode. In FIG. 4, the LDO currentsense 111 includes a transistor 401. The transistor 401 has an identicalgate to the output pass device 119, but with a size that is muchsmaller. The drain of this current sense transistor is connected to aknown current source. The size of the current sense transistor isproportional to the pass transistor and the value of the current sourceis the same proportion to the “high current” current limit. A comparatorcompares the voltage at both drains and when the OUT pin drops lowerthan the current sense then the LDO is said to be in a high currentstate.

The above detailed descriptions of embodiments of the invention are notintended to be exhaustive or to limit the invention to the precise formdisclosed above. While specific embodiments of, and examples for, theinvention are described above for illustrative purposes, variousequivalent modifications are possible within the scope of the invention,as those skilled in the relevant art will recognize. For example, whilesteps or components are presented in a given order, alternativeembodiments may perform routines having steps or components in adifferent order. The teachings of the invention provided herein can beapplied to other systems, not necessarily the network model describedhere. The elements and acts of the various embodiments described abovecan be combined to provide further embodiments and some steps orcomponents may be deleted, moved, added, subdivided, combined, and/ormodified. Each of these steps may be implemented in a variety ofdifferent ways. Also, while these steps are shown as being performed inseries, these steps may instead be performed in parallel, or may beperformed at different times.

Unless the context clearly requires otherwise, throughout thedescription and the claims, the words “comprise,” “comprising,” and thelike are to be construed in an inclusive sense as opposed to anexclusive or exhaustive sense; that is to say, in the sense of“including, but not limited to.” Words in the above detailed descriptionusing the singular or plural number may also include the plural orsingular number respectively. Additionally, the words “herein,” “above,”“below,” and words of similar import, when used in this application,shall refer to this application as a whole and not to any particularportions of this application. When the claims use the word “or” inreference to a list of two or more items, that word covers all of thefollowing interpretations of the word: any of the items in the list, allof the items in the list, and any combination of the items in the list.

The teachings of the invention provided herein could be applied to othersystems, not necessarily the system described herein. These and otherchanges can be made to the invention in light of the detaileddescription. The elements and acts of the various embodiments describedabove can be combined to provide further embodiments.

All of the above patents and applications and other references,including any that may be listed in accompanying filing papers, areincorporated herein by reference. Aspects of the invention can bemodified, if necessary, to employ the systems, functions, and conceptsof the various references described above to provide yet furtherembodiments of the invention.

These and other changes can be made to the invention in light of theabove detailed description. While the above description details certainembodiments of the invention and describes the best mode contemplated,no matter how detailed the above appears in text, the invention can bepracticed in many ways. Details of the network model and itsimplementation may vary considerably in their implementation details,while still being encompassed by the invention disclosed herein. Asnoted above, particular terminology used when describing certainfeatures, or aspects of the invention should not be taken to imply thatthe terminology is being re-defined herein to be restricted to anyspecific characteristics, features, or aspects of the invention withwhich that terminology is associated. In general, the terms used in thefollowing claims should not be construed to limit the invention to thespecific embodiments disclosed in the specification, unless the aboveDetailed Description section explicitly defines such terms. Accordingly,the actual scope of the invention encompasses not only the disclosedembodiments, but also all equivalent ways of practicing or implementingthe invention under the claims.

While certain aspects of the invention are presented below in certainclaim forms, the inventors contemplate the various aspects of theinvention in any number of claim forms. Accordingly, the inventorsreserve the right to add additional claims after filing the applicationto pursue such additional claim forms for other aspects of theinvention.

1. A DC/DC voltage regulator that receives an input voltage and outputsa stable voltage onto an output node for use by a load, the regulatorcomprising: a linear regulator controlled by a linear drop out (LDO)feedback signal, said linear regulator operative if said load is drawingless than a first predetermined amount of current; a switch-moderegulator controlled by a switch-mode (PWM) feedback signal, saidswitch-mode regulator operative if said load is drawing more than asecond predetermined amount of current; and a single feedback networkthat provides an output node feedback signal indicative of the voltageon said output node, said feedback network including a LDO erroramplifier and a PWM error amplifier, said LDO error amplifier outputtingsaid LDO feedback signal based upon said output node feedback signal andsaid PWM error amplifier outputting said PWM feedback signal based onsaid output node feedback signal.
 2. The regulator of claim 1 whereinsaid first and second predetermined amounts of current are the same. 3.The regulator of claim 1 wherein said linear regulator and said PWMregulator are in parallel with each other.
 4. The regulator of claim 1further including a linear current sensor for sensing the currentflowing through said linear regulator.
 5. The regulator of claim 1further including a switch-mode current sensor for sensing the currentflowing through said switch-mode regulator.
 6. A method of receiving aninput voltage and providing a stable voltage for use by a load, themethod comprising: providing a linear regulator controlled by a lineardrop out (LDO) feedback signal, said linear regulator operative if saidload is drawing less than a first predetermined amount of current;providing a switch-mode regulator controlled by a switch-mode (PWM)feedback signal, said switch-mode regulator operative if said load isdrawing more than a second predetermined amount of current; andproviding a single feedback network that provides an output nodefeedback signal indicative of the voltage on said output node, saidfeedback network including a LDO error amplifier and a PWM erroramplifier, said LDO error amplifier outputting said LDO feedback signalbased upon said output node feedback signal and said PWM error amplifieroutputting said PWM feedback signal based on said output node feedbacksignal.
 7. The method of claim 6 wherein said first and secondpredetermined amounts of current are the same.
 8. The method of claim 6wherein said linear regulator and said PWM regulator are in parallelwith each other.
 9. A method of receiving an input voltage and providinga stable voltage for use by a load, the method comprising: using alinear regulator controlled by a linear drop out (LDO) feedback signalto provide said stable voltage to said load if said load is drawing lessthan a first predetermined amount of current; using a switch-moderegulator controlled by a switch-mode (PWM) feedback signal to providesaid stable voltage to said load if said load is drawing more than asecond predetermined amount of current; and feeding back through asingle feedback network an output node feedback signal indicative of thevoltage on said output node, said feedback network including a LDO erroramplifier and a PWM error amplifier, said LDO error amplifier outputtingsaid LDO feedback signal based upon said output node feedback signal andsaid PWM error amplifier outputting said PWM feedback signal based onsaid output node feedback signal.
 10. The method of claim 9 wherein saidfirst and second predetermined amounts of current are the same.
 11. Themethod of claim 9 wherein said linear regulator and said PWM regulatorare in parallel with each other.